1. Field of the Invention
This invention relates to protection controls, and more particularly to a differential pressure sensing apparatus for monitoring two pressures and for providing a control output whenever the difference between the pressures reaches a preselected value.
2. Description of the Prior Art
Proper lubrication is a major concern in machines with high-speed moving parts or with heavy loading of bearings or both. Accordingly, lubrication oil protection controls have become widely used in apparatus, such as compressors for refrigeration and air conditioning systems, to detect the condition of inadequate lubrication and effect shut down of the compressor for such conditions.
The need for such lube oil protection controls is readily apparant when one considers that the compressors are usually required to run hour after hour, day after day, unattended, and without such controls, continued operation of a compressor with improper lubrication would result in damage to the compressor.
One reason for low oil pressure is that if there is no oil in the compressor crankcase, the oil pump cannot deliver oil to the bearings and other wearing surfaces. Other common causes of low oil pressure include the presence of liquid refrigerant in the compressor crankcase, oil pump failure, or an inoperative pressure relief valve or pressure regulating valve in the lubrication oil system, for example.
Accordingly, pressure sensing controls have been developed for the purpose of monitoring oil pressures for compressors of refrigeration or air conditioning units and the like.
Since the refrigeration system is a sealed system, then when the refrigeration compressor is idle, the oil pump pressure and the compressor crankcase oil pressure are generally the same. When the compressor is running, the oil pump pressure increases and the crankcase oil pressure decreases, and only if the lubrication system is not performing properly will these two pressures remain the same or nearly the same with the compressor running.
The difference between these two pressures, commonly termed "differential pressure" is usually monitored by a differential pressure sensing device as a way of determining if proper lubrication is provided for the compressor. Typically, such device responds to a differential pressure at least equal to a set value to maintain the compressor operating. The device responds to a differential pressure below the set value to effect the operation of a switch which causes the deenergization of the compressor, such as by de-energizing the coil of a motor contactor.
However, once an under pressure condition is sensed, such thermal switches require a time delay, typically one or two minutes before effecting deactivation of the compressor. Also, the timing is affected by ambient temperatures and voltage variations.
Moreover, in recent years, with the development of smaller, more compact compressors and motor compressor assemblies, there has been a demand for smaller, more compact controls which provide such differential pressure monitoring function.